I did not vote to close this question. However, I think that I understand the motivation behind it.
There is a school of belief that questions about why a politician chose to do something is pure opinion. The only person who knows is the politician, and even the politician might be confused as to the basis. People aren't always aware of all their own motivations. Still, the best answers are often those where the politician has publicly given a reason which can be linked and quoted.
These questions tend to be answered with speculation. This speculation can be based on facts or it can be personal rants. So questions like this tend to attract answers that get deleted. This is discussed in the on-topic help:
In just about any policy of substance, there are particular personalities that are central to its understanding, as well as demographic data about supporters and opponents of legislation. Asking “Why is [insert person here] such a jerk?” is clearly off-topic - the answer is highly subjective, but asking “What groups of people tend to support X in her implementation of policy Y?” is answerable using polls, punditry, and other verifiable and reproducible sources.
If your question (or answer!) is subjective, opinionated, or just not about one of these three things, we humbly suggest you look elsewhere for an answer to your query.
It should be rather obvious that this does fit under the third pillar. However, note the subjective exception. Some people simply find such questions to be to subject to badly based answers.
My own view is that questions like this are more useful than not. They help capture multiple viewpoints about country-specific questions. In this example, many of us probably weren't aware of either of those parties prior to this question. So a decent explanation would probably tell us a lot about politics in Australia that we didn't know previously.